ELECTRICITY 
AT    HIGH    PRESSURES 


BY 

ELIHU  THOMSON 


A  Lecture  delivered  before  the  New  York  Electrical  Society 

March  29,  1899 
At  the  Society  of  Civil  Engineers,  New  York  City 


£  \  Vcvn^^cfc 

o 


ELECTRICITY 
AT    HIGH    PRESSURES 


BY 

ELIHU   THOMSON 


A  Lecture  delivered  before  the  New  York  Electrical  Society 

March  29,  1899 
At  the  Society  of  Civil  Engineers,  New  York  City 


ELECTRICITY  AT   HIGH    PREgSlIRES. 

MY  interest  in  the  subject  we  shall  consider  this  evening  .is  thte 
more  pronounced  from  the  fact  that  I  began  my  ex'peji^)ice,\\;it:K 
electrical  phenomena  by  working  with  relatively  high  pressures  or 
potentials.  My  first  machine,  constructed  when  I  was  eleven  years 
of  age,  was  a  high  potential  apparatus,  giving  about  30,000  volts  or 
thereabouts.  It  was  a  frictional  electric  machine,  the  main  part 
being  a  wine-bottle  revolved  upon  an  axis,  to  which  was  added  the 
usual  rubber,  silk  flap,  and  prime  conductor. 

Electrical  development  on  the  large  scale  has  in  the  past  few 
few  years  been  going  on  in  the  direction  of  increase  of  electrical 
pressures,  or  increase  of  potential  differences,  and  this  fact  gives 
a  renewed  interest.  The  prime  cause  of  all  electrical  manifesta- 
tions is,  of  course,  difference  of  pressure  or  potential.  That  we 
have  much  more  to  learn  in  this  fascinating  field  is  evidenced  by 
the  condition  of  our  knowledge  in  regard  to  the  phenomena  of 
lightning,  the  aurora,  comets'  tails,  and  possibly,  also,  of  the  solar 
corona.  It  is  about  one  hundred  years  since  Volta  studied  the 
voltaic  battery  and  gave  to  the  world  a  source  of  steady  currents 
at  low  pressure.  Long  before  that,  however,  the  older  experi- 
ments of  the  development  of  electric  charges  by  friction,  and  the 
properties  of  charged  bodies  had  been  studied  and  wondered  at. 

Priestley,  in  his  "History  of  Electricity,"  a  work  of  nearly  800 
pages,  has  even  given  expression  to  the  opinion  that,  "In  elec- 
tricity there  is  the  greatest  room  to  make  new  discoveries.  It  is 
a  field  but  just  opened,"  etc.  His  book  was  published  in  the  latter 
part  of  the  last  century.  It  dealt  entirely  with  electricity  at  high 
pressures.  Even  the  old  experiment  of  rubbed  amber  would  give 
10,000  to  20,000  volts,  and  the  old  glass  globe  machines,  such  as 
Franklin  used,  a  much  higher  pressure;  nothing,  however,  in  com- 
parison with  that  of  a  lightning  stroke.  Still  I  must  say  that  I 
have  no  sympathy  for  those  who  speak  without  hesitation  of  hun- 
dreds of  millions  of  volts  even  in  such  cases,  or  who  affect  to 
describe  apparatus  as  in  existence  to-day  developing  pressures  of 
several  millions  of  volts.  A  few  years  ago  (in  1892)  I  constructed 
a  high  potential,  high  frequency  coil  which  gave  a  torrent  of  64-inch 


679936 


4  ELECTRICITY  AT   HIGH    PRESSURES. 

sparks,  estimated  by  me  at  the  time  as  representing  about  three- 
fourths  of- a  million  volts,  and  since  thought  by  Professor  Trowbridge 
of  Jefferson  Physical  Laboratory,  Harvard  University,  to  have 
required  one  million  volts.  Professor  Trowbridge  has  since  con- 
structed apparatus  from  which  he  has  obtained  some  sparks  of 
seven  feet  in  length,  a  discharge  of  perhaps  something  more  than  a 
million  and  a 'half  volts. 

This  apparatus  was    calculated  to  give  about    three  millions    of 
*vci'ts,'but  proba'bly-  before  that  pressure  was  obtained,  the  leakage, 
brush  discharges  to  the  walls,  floor,  and  surrounding  objects,  limited 
the  pressure  to  that  which  gave  the  discharge  of  seven  feet. 

To  illustrate  the  development  of  electrical  pressures  by  friction, 
I  may  describe  an  experiment  of  many  years  ago.  I  poured  a  layer 
of  turpentine  varnish  upon  a  clean  tin  plate  and  allowed  it  to  dry 
into  a  flexible  sheet  closely  adhering  to  the  tin.  Upon  stripping 
it  off  carefully  so  as  not  to  cause  friction,  it  came  away  with  an 
electrical  pressure,  or  charge,  sufficient  to  cause  small  sparks,  etc. 
The  varnish,  in  close  association  with  the  tin  surface,  had  taken  up 
an  opposite  electrical  state  to  that  of  the  tin.  The  pressure  or 
potential  difference  was  not,  however,'  high  while  the  surfaces- 
remained  in  contact,  but,  upon  separating  them,  the  pressure  rose 
gradually  during  separation.  This  was  owing  to  lessened  capacity,, 
there  being  no  more  electricity  in  the  one  case  than  in  the  other. 
The  positive  and  negative  electricities  were  neutralized  or  bound 
by  being  close  together,  and  the  pressure  was  very  low.  When 
the  film  was  raised,  the  electrical  pressure  rose  and  energy  was 
expended,  just  as  it  is  expended  in  lifting  a  weight. 

I  may  illustrate  the  conditions  by  laying  a  thin  sheet  of  rubber 
upon  a  smooth  zinc  plate,  and  rubbing  with  a  piece  of  fur  the  upper 
surface  of  the  thin  rubber  film.  A  bound  charge  is  accumulated 
upon  the  rubber,  and  the  zinc  and  the  rubber  exhibit,  as  tested  by 
the  electroscope,  only  a  slight  electrification.  The  charge  is  so 
nearly  neutralized  by  the  opposite  charge  brought  up  by  the  zinc 
that  the  potential  exhibited  is  very  low.  Upon  stripping  the  thin 
rubber  from  the  plate,  work  is  done  in  overcoming  the  attraction 
due  to  the  opposite  charges,  and  the  potential  of  the  rubber  at 
once  rises  so  that  it  gives  sparks  of  two  or  three  inches  to  the 
knuckle  held  near  it. 

When,  as  in  this  case,  the  electricity  is  developed  by  friction, 
the  rubbing  only  acts  to  secure  area  of  contact,  or  to  insure  the 
condition  that  all  parts  of  the  rubbed  surface  shall  have  been  in 
close  contact  with  the  rubbing  body — in  this  case  the  fur.  If  a 


ELECTRICITY   AT   HIGH    PRESSURES.  5 

piece  of  ebonite  be  rubbed  by  the  fur,  wherever  the  two  touch 
bound  charges  are  developed  and  would  remain  bound  and  neutral- 
ized. They  would  exist  at  low  voltage  until  they  were  separated. 
The  high  pressure  charges  only  come  into  existence  as  the  two 
bodies  are  gradually  separated.  How  inefficient  the  friction  process 
must  be  can  be  understood  when  it  is  pointed  out  that  the  actual 
work  which  produces  electricity  is  not  that  of  the  rubbing,  but 
only  that  of  the  contact  and  separation  of  the  two  bodies.  The 
friction  proper  produces  heat;  the  pulling  apart  of  the  bound 
-charges  gives  electrical  energy  at  high  pressure. 

The  phenomena  of  development  of  high  electrical  pressures  by 
belts,  such  as  those  of  high  speed  machinery,  are  illustrations  of 
the  same  principles.  Instructions  are  sometimes  given  to  discharge 
the  belt  by  a  row  of  points  connected  to  earth,  so  as  to  avoid  the 
belt  electricity  endangering  the  insulation  of  a  dynamo  or  motor  by 
-charging  its  frame.  Such  a  procedure  is  the  very  one  to  enhance 
or  increase  such  danger,  and  the  proper  procedure  is  to  connect 
the  points  to  the  frame  itself  and  to  the  shaft  which  serves  as  the 
•countershaft  belted  to  the  dynamo  shaft.  Some  of  the  larger  belts 
used  in  transmission  give  very  striking  exhibitions  of  electricity 
at  high  potentials.  Sparks  over  three  feet  long  were  obtained  under 
a  large  belt  at  the  Narragansett  Electric  Lighting  Company's 
station,  in  Providence,  some  years  ago. 

The  old  electrophorus  was  an  instance  of  the  utilization  of 
similar  principles  of  alternate  binding  and  freeing  of  a  charge  pro- 
duced by  friction  upon  a  resinous  plate  contained  in  a  sheet  metal 
tray.  When  the  insulated  disk  of  the  electrophorus  is  lifted  off  the 
resin  surface,  the  charge  on  the  resin  binds  itself  to  an  opposite 
charge  induced  in  the  tray,  and  when  the  disk  is  returned  and  con- 
nected to  the  tray  or  to  ground,  the  charge  mostly  binds  itself  with 
an  opposite  charge  induced  in  the  disk  now  resting  upon  the  rubbed 
resin.  Upon  disconnecting  the  disk  from  earth  and  lifting  it  off 
the  resin  surface,  it  is  found  to  be  possessed  of  a  charge  of  opposite 
name  to  that  upon  the  resin  surface,  which  latter  again  binds  itself 
to  its  opposite  in  the  tray.  The  work  done  is  the  lifting  off  of  the 
disk  against  electrical  attraction  between  the  disk  and  resin  surface, 
the  energy  used  in  lifting  manifesting  itself  in  a  high  pressure 
•charge  in  the  disk. 

The  apparatus  known  as  Armstrong's  Hydro-Electric  Machine 
is  an  instructive  example  of  the  operation  of  similar  principles. 
It  enables  a  high  potential  to  be  produced  from  the  effects  of  a 
slight  charge  produced  in  minute  drops  of  water  which  are  driven 


ELECTRICITY   AT   HIGH    PRESSURES. 


along  with  steam,  through  a  jet  lined  with  box-wood.  The 
globules  from  partly  condensed  steam,  on  leaving  the  jet  form  a 
small  cloud  into  which  collecting  points  from  a  prime  conductor 
project,  and  a  high  potential  charge  is  thus  given  to  the  con- 
ductor, from  which  long  sparks  may  be  taken.  Since  the  actions 
occurring  in  this  steam  electrical  machine  serve  to  throw  light  on 
the  accumulation  of  high  pressures  in  a  thunder  cloud,  I  may  be 
pardoned  for  dwelling  upon  the  matter  for  a  few  minutes. 

Let  S,  Fig.  i,   represent  an    insulated  sphere,    charged    to,    say, 


e%S:>e 

000       °  0 

0      0      °     00 
©        ©    0 


0©    ©0 


FIG.  i. 


FIG.  2. 


10,000  volts  potential  with  respect  to  earth.  It  will  have  a  certain 
capacity  depending  upon  the  proximity  of  surrounding  conducting 
objects. 

Let  this  sphere  be  now  surrounded  by  a  number  of  hollow 
spheres,  such  as  by  bringing  hollow  hemispheres  together  over  the 
center  sphere,  properly  supported  and  insulated.  Let  the  hollow 
spheres  be  well  insulated  from  S,  and  from  each  other,  by  a  sufficient 
layer  of  air  or  dielectric.  If  each  hollow  sphere  be  like  S,  charged 
to,  say,  10,000  volts  before  being  brought  over  S,  the  potential  ex- 
hibited by  the  outer  sphere  will  gradually  rise  as  the  spheres  are 
added.  This  is  owing  to  the  diminished  capacity  of  the  concentric 
shells  as  compared  with  their  capacity  when  separately  charged. 

If  we  now  substitute  for  the  hollow  spheres  and  central  sphere 
a  large  number  of  floating  water  globules  in  air  as  electrified  and 
driven  out  from  a  jet  J,  Fig.  2,  of  a  Hydro-Electric  Machine,  we 
have  a  similar  condition,  the  result  of  which  is  that,  although  the 


ELECTRICITY  AT   HIGH    PRESSURES.  7 

«• 

globules  so  driven  together  in  a  mass  but  are  slightly  charged  when 
leaving  the  jet,  the  combined  effect  of  all  results  in  greatly  en- 
chanced  potential,  owing  to  the  fact  that  the  water  globules  within 
the  small  cloud  have  virtually  been  deprived  of  capacity.  Add  to 
this  the  effect  of  diminished  total  surface  of  the  globules  due  to 
their  coalescing,  and  further  the  reduction  of  the  surface  of  the 
globules  themselves  by  gradual  evaporation  in  the  air,  and  we  need 
not  be  surprised  that  the  potential  shown  by  the  mass  is  high,  though 
it  was  formed  by  driving  forward  into  the  cloud  globules  originally 
at  comparatively  low  potentials. 

Applying  the  same  considerations  to  the  thundercloud  itself,  we 
seem  to  understand  why  it  is  that  globules  of  water  suspended  in 
air  may  act  inductively,  when  only  slightly  electrified  at  the  start, 


FIG.  3. 

and  so  give  rise  to  the  exhibition  of  enormous  potentials.  Assume 
a  horizontal  layer  of  air  L,  Fig.  3,  saturated  with  moistureand  possess- 
ing an  electrical  charge,  however  it  may  have  been  accumulated.  If 
such  a  layer  be  uplifted  by  warm  uprising  currents  from  below,  it  will 
condense  into  a  cumulus  cloud  containing  electrified  globules  of 
water  insulated  from  each  other,  but  massed  together  in  the  cloud 
with  diminishing  capacity  due  to  the  various  causes  before  men- 
tioned: the  inductive  effect  outward  of  the  globules  within  the 
mass;  the  coalescence  of  numerous  small  globules  into  larger  ones; 
and  possibly  also,  in  some  portions  of  the  cloud,  the  diminishing  size 
due  to  evaporation.  The  cloud,  as  a  whole,  will  show  a  high  state 
of  electrification,  but  no  one  part  of  it  need  at  any  time  be  very 
highly  charged  in  the  sense  that  an  insulated  body  is  charged.  In 


8  ELECTRICITY   AT   HIGH    PRESSURES. 

the  latter  the  charge  is  on  the  surface.  In  the  cloud  each  globule 
is  charged  and  also  acts  inductively  with  its  neighbors,  outwardly 
and  toward  the  earth.  In  discharging,  the  spark  forks  and  extends 
or  ramifies  through  the  cloud  masses,  often  occupying  an  appreciable 
time  in  this  process.  The  discharge  of  one  portion  of  cloud  opens 
a  good  path  of  hot  gas  for  other  discharges  to  follow,  and  the 
disturbance  thus  spreads.  Frequently  discharges  are  multiple  in 
character,  several  successive  discharges  following  the  same  path  to 
earth  under  the  cloud,  but,  doubtless,  within  the  cloud,  ramifying 
in  different  directions  and  discharging  different  portions  of  the 
cloud  mass. 

If  this  idea  of  the  actual  conditions  which  exist  in  a  thundercloud 
be  true,  it  may  easily  be  understood  that  attempts  to  represent  the 
action  of  lightning  by  the  discharge  of  small  condensers,  such  as 
Leyden  Jars,  may  utterly  fail  of  their  purpose.  Neither  can  any 
arguments  avail,  based  upon  the  discharges  of  lightning  having  a 
rapid  oscillatory  character.  That  the  breakdown  of  the  air  under  a 
thundercloud  when  a  stroke  to  earth  occurs  is  very  sudden  is 
doubtless  true,  but  that  the  flow  or  flash  is  oscillatory  or  that  it  en- 
dures for  an  excessively  short  time  may  not  be  true;  at  least,  not  in 
all  cases.  The  conditions  must  be  very  variable,  and  the  discharges, 
occurring  under  wide  variations  of  these  conditions,  are  not  likely 
to  be  alike.  It  is  useless,  therefore,  to  attempt  to  calculate  the 
voltage  or  the  current  in  a  flash  of  lightning,  and  no  estimate  of  the 
energy  expended  can  be  more  than  a  guess. 

Such  devices  as  lightning  arresters  cannot  be  fully  tested  as  to 
their  action  or  effectiveness  without  actual  practice  with  them  dur- 
ing thunderstorms,  and  tests  made  with  ordinary  static  discharges 
from  Leyden  Jars  may  not,  and  probably  do  not,  represent  the  real 
conditions  under  which  the  devices  themselves  will  be  required 
to  work.  Hertz  showed  that  the  higher  pitch  rays,  such  as  those 
of  the  violet  or  ultra  violet  order  in  the  spectum,  served  to  precipi- 
tate electric  discharges  between  terminals  set  so  far  apart  that  the 
potential  between  them  was  insufficient  to  cause  a  spark.  The 
thought  occurs  that  possibly  such  phenomena  as  that  of  the  return 
stroke,  as  well  as  that  of  multiple  strokes  or  flashes  of  lightning,  oc- 
curring practically  simultaneously  over  different  paths,  may  some- 
times depend  on  the  sudden  illumination  of  the  air  by  a  first  discharge 
provoking  other  discharges  around  it. 

I  am,  moreover,  led  to  suspect  that  very  long  discharges  in  cloud, 
such  as  those  which  occur  horizontally,  over  distances  of  perhaps 
five  to  ten  miles  in  a  cloud  layer,  actually  take  an  appreciable  time 


ELECTRICITY   AT   HIGH    PRESSURES.  9 

to  develop  or  progress.  There  seems  indeed  to  be  a  progressive 
breakdown  of  the  cloud,  extending  on  and  on  during  a  time  which 
can  be  noted. 

I  have  in  fact  often  watched  the  development  of  such  discharges 
in  the  east  or  toward  the  eastern  horizon,  and  have  been  able  to 
turn  the  head  and  follow  the  discharge  to  its  finish  in  the  west  or 
toward  the  western  horizon.  I  have  noticed  also  that  these  long 
discharges  will  start  as  a  single  streak,  ramify,  and  extend  their  rami- 
fications before  the  eyes.  I  am  persuaded  that  this  is  no  optical 
illusion;  the  effect  seems  too  definite  for  that.  It  would  be  of  deep 
interest  to  photograph  such  discharges  on  two  plates,  one  fixed  and 
the  other  revolving,  and  compare  the  images.  This  is  quite  a  diffi- 
cult matter,  however,  as  lenses  having  a  sufficiently  wide  angle  to 
take  in  the  whole  path,  or  practically  the  whole  sky  at  once,  are  not 
to  be  had,  and  one  never  knows  just  when,  and  at  what  portion  of 
the  cloud,  the  desired  discharge  may  take  place.  Moreover,  for 
such  work  the  possibility  of  pointing  a  camera  upward  or  nearly 
vertical  may  be  prevented  by  rain  occurring  at  the  time. 

How  much  greater  is  the  contrast  that  existed  in  Franklin's  time 
between  the  lightning's  effects  and  those  of  the  feebly  acting  ap- 
paratus at  his  command,  than  can  be  said  to  exist  to-day  with 
modern  apparatus  capable  of  yielding  long  sparks!  With  the  latter, 
Franklin's  genius  would  not  have  been  needed  to  discern  electricity 
in  lightning. 

The  old  frictional  machines  possess  now  only  an  historical  inter- 
est. The  principle  of  the  electrophorus  was  applied  about  1866  by 
Holtz  in  his  famous  influence  machine,  which  has  undergone  various 
modifications  from  time  to  time  during  the  past  thirty  years.  It  is 
not  necessary  to  discuss  the  details  of  its  construction  and  the 
theory  of  operation.  The  various  forms  of  static  influence  ma- 
chines are  in  reality  forms  of  continuous  electrophorus,  the  more 
recent  machines  embodying  the  principle  of  the  multiplier  or 
doubler  used  by  Lord  Kelvin  many  years  ago  in  charging  his 
electrometers.  A  very  large  machine  has  recently  been  constructed 
under  the  direction  of  Dr.  Francis  H.  Williams  of  the  City  Hos- 
pital, Boston,  for  use  in  exciting  Crookes  tubes  in  his  Roentgen- 
ray  work.  It  has  four  revolving  glass  plates  six  feet  in  diameter, 
and  the  effects  obtainable  are  very  excellent  for  Roentgen-ray 
excitation. 

We  have  before  us,  as  a  loan  for  this  lecture,  a  splendid  example 
of  a  Holtz  influence  machine  as  manufactured  by  the  Galvano-Faradic 
Mfg.  Company  of  New  York.  It  has  eight  revolving  glass  plates 


10  ELECTRICITY   AT    HIGH    PRESSURES. 

of  30  inches  in  diameter,  in  addition  to  a  pair  of  plates  which  can  be 
revolved  oppositely  by  hand  for  charging  the  sectors  of  the  main 
machine.  It  is  now  driven  by  an  electric  motor  of  1/6  H.  P.,  and 
works  with  great  smoothness.  The  construction  of  the  machine  is 
certainly  a  credit  to  its  makers,  who  have  apparently  given  careful 
attention  to  every  detail.  They  have  produced,  also,  a  very  hand- 
some piece  of  electrical  apparatus.  The  foregoing  apparatus  de- 
pends chiefly  upon  the  principle  of  increasing  potential  by 
diminishing  capacity.  If  a  number  of  Leyden  jars,  or  other  con- 
densers of  nearly  equal  capacity,  be  charged  in  multiple  and  then 
connected  in  cascade  or  in  series,  the  terminal  difference  of  pressure 
or  potential  will  be  found  to  be  increased  in  proportion  to  the  num- 
ber of  jars  so  connected.  In  this  case  the  capacity  of  the  set,  when 
in  multiple  and  acting  as  one  large  condenser,  is  virtually  reduced  at 


FIG.  4. 

the  moment  of  connection  in  series  and  the  pressure  of  the  con- 
denser rises  accordingly.  This  is  the  principle  of  the  Plante  rheo- 
static  machine,  to  be  alluded  to  later. 

To  illustrate  the  actual  change  which  takes  place  in  the  Rheo- 
static  machine,  we  may  represent  by  A,  Fig.  4,  four  condensers 
charged  in  parallel.  The  single  thickness  of  dielectric  is  equally 
strained;  there  being  four  units  of  surface  -j-  and  — .  When  these 
are  connected  in  series  the  end  foils  only  need  to  be  considered: 
the  dielectric  becomes  four  times  as  thick,  as  shown  at  B,  and  the 
dielectric  is  throughout  under  the  same  electric  stress  as  before. 
Such  a  condition  cannot,  however,  be  maintained  without  an  in- 


ELECTRICITY  AT   HIGH    PRESSURES.  II 

crease  of  potential  difference  between  the  single  pair  of  condenser 
foils  in  B.  There  is  thus  a  multiplied  potential  according  to  the 
number  of  plates  so  placed  in  series. 

Besides  the  method  of  raising  potential  by  diminishing  capacity 
as  above,  electromagnetic  methods  are  well  known  and  are  now  em- 
ployed extensively.  When  a  coil  of  wire  is  wound  upon  an  iron  core 
and  a  second  coil  of  finer  wire  is  wound  alongside  of  it  or  around 
the  same  iron  core,  a  periodic  current  sent  through  the  first  wind- 
ing induces  a  periodic  electromotive  force  wave,  or  a  current,  as 
the  case  may  be,  representing  an  increase  of  pressure  or  potential 
nearly  in  the  relation  of  the  turns  in  each  coil.  This  relation  is 
called,  in  alternating  current  transformers,  the  transforming  ratio. 
In  high  potential  transmission  this  ratio  maybe  even  as  great  as  200 
to  i  in  extreme  cases.  Usually  the  ratio  is  not  so  great.  The 
same  transformers,  being  completely  reversible  in  function,  are 
used  both  for  step  up  and  step  down  transformation. 

A  very  large  amount  of  power  is  now  transmitted  over  varying 
distances  by  utilizing  this  principle  of  transformation.  Many  large 
plants  are  in  operation,  using  for  the  line  6000  to  10,000  volts; 
some  involve  pressures  of  20,000,  and  one  plant  in  California  uses 
40,000  volts  on  the  line.  This  last  is,  properly  speaking,  a  very 
high  pressure  for  such  work.  Whether  it  will  be  much  exceeded  in 
the  further  development  of  the  art  remains  to  be  seen.  Mr.  Chas. 
F.  Scott  in  a  recent  paper  has  ably  dealt  with  the  question  of  in- 
crease of  voltage  in  systems  of  transmission,  and  the  subject  is  too 
extended  to  be  taken  up  in  detail  now.  Suffice  it  to  say  that  there 
are  indications  of  a  limit  being  reached  at  which  the  losses  from  the 
wires  may  become  so  great  as  to  neutralize  the  benefits  attained  in 
raising  potential.  Besides,  the  capacity  of  a  very  long  line  at  high 
pressures  introduces  new  problems  and  requires  additional  compli- 
cation to  balance  it.  We  have  very  much  to  learn  in  the  field  of 
high  pressure  transmissions.  Difficulties  which  seem  insurmount- 
able to-day  may  disappear  in  the  technical  advance  of  a  few  years. 
This  has  always  been  the  case  in  the  development  of  new  arts,  and 
it  would  be  unwise  at  present  to  assign  any  limits  to  increase  of 
pressure. 

Back  in  1878  I  used  to  wonder  whether  I  should  live  to  see  elec- 
tric currents  used  of  as  much  as  25  to  50  amperes  with  a  line  of  a 
potential,  or  tension  as  we  then  called  it,  such  that  a  discharge 
would  leap  from  the  line  to  earth  over  as  much  as  half  an  inch.  I 
discussed  the  possibility  of  such  transmissions  with  friends,  and  tried 
to  picture  to  myself  the  conditions  of  such  a  line,  even  figuring  out 


12  ELECTRICITY   AT   HIGH   PRESSURES. 

the  energy  transference.  I  am  free  to  confess  that  I  had  little 
hope  of  eve.r  being  a  witness  to  the  actual  realization  of  such 
dreams.  But  we  are  indeed  far  beyond  those  early  meditations  in 
our  actual  accomplishment.  We  build  transformers  for  10,000  to 
30,000  volts  of  pressure  and  send  hundreds  of  amperes  into  the  line 
at  such  pressures;  and  we  transmit  power  in  thousands  of  kilowatts 
over  distances  up  to  eighty  miles.  Transformers  are  built  which 
singly  have  a  capacity  of  2000  kilowatts,  and  10,000  volts  are  em- 
ployed on  lines,  as  at  Niagara,  where  the  distance  is,  say,  two  and  a 
half  miles  only,  because  copper  can  be  saved.  A  transformer  of 
such  large  capacity  as  to  have  a  voltage  of  sixty  for  each  turn 
around  its  core  no  longer  surprises  us;  and  transmissions  of  power 
involving  15,000  to  20,000  horse  power,  to  use  the  old  unit  now 
gradually  being  displaced  by  the  kilowatt,  are  beginning  to  be  ordi- 
nary achievements  of  electrical  engineering. 

The  potential  which  is  developed  in  any  coil  through  the  axis  of 
which  a  magnetic  circuit  is  or  can  be  formed  will  depend  upon  the 
maximum  rate  of  change  in  the  number  of  lines  of  force  of  such 
circuit,  in  entering  or  leaving  the  core.  Each  line  of  force  will,  in 
being  introduced  or  removed  from  the  area  inclosed  by  the  wire 
coil,  develop  electromotive  force  in  each  turn  so  cut  by  the  mag- 
netic line,  which  electromotive  force  depends  on  the  rate  of  cutting. 
We  of  course  use  the  phrase  "line  of  force  "  as  a  convenience. 

The  first  contact,  so  to  speak,  between  the  old  so-called  statical 
electricity  or  electricity  of  "high  tension"  and  "galvanic  or 
voltaic  electricity,"  which  were  at  one  time  regarded  as  distinct 
species,  was  made  by  the  invention  of  the  Ruhmkorff  coil  or  induc- 
tion coil.  It  bridged  the  gap  existing,  and  served  to  break  down  a 
barrier  and  destroy  distinctions  between  different  kinds  of  elec- 
tricity, which  distinctions  are  never  entertained  now. 

In  such  an  ordinary  induction  coil  used  for  obtaining  high 
pressure  sparks  or  discharges  from  low  pressure  currents,  as  of  a 
few  cells  of  battery  in  series,  the  ratio  of  transformation  is  made 
very  high  and  the  rate  of  change  of  the  magnetism  of  the  field  or 
iron  core  is  also  made  as  great  as  possible.  The  turns  of  the  sec- 
ondary coil  may  bear  to  those  of  the  primary  the  relation  of  1000 
to  i,  and  the  interruption  of  the  current  sent  through  the  primary 
is  made  as  sudden  as  possible.  The  simple  addition  of  a  condenser 
across  the  break  in  the  primary  circuit  greatly  increased  the 
potential  obtainable  from  a  given  sized  coil.  The  condenser  not 
only  prevents  spark  at  the  contacts  of  the  interrupter  of  the  coil, 
which  spark  would  prolong  the  current  and  make  the  interruptions. 


ELECTRICITY   AT   HIGH   PRESSURES.  13 

much  less  sudden,  but  it  also  acts  to  receive  the  self-inductive  dis- 
charge of  the  circuit  or  extra  current  of  Faraday,  and  becomes  itself 
so  highly  charged  that  it  can  send  a  reversed  current  through  the 
primary  circuit,  and  so  increase  the  rate  and  extent  of  change  of 
magnetism  of  the  core  upon  which  the  potential  developed  in  the 
secondary  largely  depends.  The  condenser  in  this  case,  in  fact, 
gives  an  oscillatory  character  to  the  primary  discharge,  as  is  easily 
noted  in  the  changing  sound  of  the  secondary  spark  when  the 
capacity  of  the  condenser  is  varied.  I  have  in  my  possession  a  coil 
which  shows  this  phenomenon  very  clearly. 

The  insulation  is  of  supreme  importance  in  these  cases  of  devel- 
opment of  very  high  electric  pressures  in  coils  of  this  type. 

A  new  interrupter,  Fig.  5,  remarkable  for  its  extreme  simplicity 
and  for  the  sharpness  of  its  interruptions,  has  been  recently 


FIG.  5. 

described  by  Dr.  Wehnelt.  As  no  condenser  is  needed  with  this 
device,  it  leads  to  a  great  simplification  of  the  coil  apparatus.  The 
interrupter  consists  simply  of  a  vessel  containing  an  electrolyte, 
such  as  sulphuric  acid  of  density  1.2,  into  which  a  lead  plate  or 
surface  is  plunged  and  connected  as  a  cathode  in  the  circuit.  Dip- 
ping into  the  liquid  is  a  glass  tube,  into  the  end  of  which  has  been 
sealed  a  platinum  wire  which  projects  into  the  liquid  a  short  dis- 


14  ELECTRICITY   AT   HIGH    PRESSURES. 

tance.  The  wire  is  attached  to  a  cable  within  the  glass  tube,  or  its 
inner  end  is  surrounded  by  mercury  into  which  a  wire  is  dipped  for 
connection  to  the  platinum.  The  platinum  is  made  an  anode.  The 
induction  coil  primary  is  connected  without  condensers,  in  series 
with  the  interrupter  across  battery  terminals  or  lighting  mains,  and 


FIG. 


the  action  is  such  that  the  current  in  the  interrupter  circuit  is 
periodically  cut  off. 

Figs.  6  and  7  are  photographs  of  the  stream  of  sparks  produced 
by  an  induction  coil  so  operated.  Fig.  7  is  in  reality  a  flaming  arc 
like  a  high  potential  alternating  current  discharge. 

I  conceive  the  action  of  the  Wehnelt  interrupter  to  be  about  as 
follows:  The  gradual  increase  of  the  current  through  the  primary 
on  closing  the  circuit  causes  the  platinum  anode  to  evolve  oxygen 
gas  bubbles,  which  tend  to  cut  down  the  section  of  the  liquid  in  con- 


FIG.  7. 

tact  with  the  anode.  The  inductance  of  the  primary  circuit  tends 
to  maintain  the  current  through  the  limited  section  of  the 
liquid  in  contact  with  the  wire.  This,  in  turn,  leads  to  more  rapid 
gas  evolution  and,  possibly,  disassociation  of  the  liquid,  whereby 


ELECTRICITY   AT   HIGH    PRESSURES.  15 

there  is  a  very  sudden  stripping  of  the  liquid  layer  from  the 
platinum  anode  surface,  which,  being  now  surrounded  only  with  gas, 
is  out  of  contact  with  the  liquid.  This  action  occurs  so  suddenly 
as  to  give  rise  to  small  but  vigorous  explosions  of  vapor  or  gas. 
The  condensation  into  liquid,  or  reunion  of  the  particles  after  this 
separation  and  rupture  of  contact  at  the  platinum  surface,  causes  a 
sudden  rush  of  the  liquid  conductor  toward  the  platinum,  re-estab- 
lishing the  circuit,  which  is  thereafter  very  quickly  interrupted  as 
at  first,  and  so  on.  The  period  of  these  interruptions  will  naturally 
be  higher  the  higher  the  potential  of  the  circuit  supplying  the  cur- 
rent, the  lower  the  inductance  of  the  circuit  of  the  interrupter,  and 
the  smaller  the  platinum  anode  surface. 

In  illustration  of  this  fact,  I  have  brought  before  you  a  small 
apparatus  (Fig.  8),  which  is  used  to  produce  varying  rates  of  inter- 
ruption. It  consists  of  a  small  interrupter  in  series  with  a  small 
coil  wound  upon  an  iron  core.  The  interrupter  is  made  of  an  ordi- 


FIG.  8. 

nary  tumbler  and  has  four  platinum  anodes  of  varying  size  dripping 
in  the  acid  solution.  I  shall  use  three  of  these  in  parallel.  Out- 
side the  small  coil  on  the  iron  core,  or  primary  coil,,  is  a  single 
layer  of  secondary  with  taps  from  each  turn,  thirty-five  taps  in  all, 
taken  to  stationary  contacts  arranged  in  an  arc  of  a  circle.  These 
are  traversed  by  a  radial  arm,  which  enables  me  to  short  circuit 
more  or  less  of  the  secondary  turns.  I  am  thus,  as  you  see, 
enabled  to  actually  .play  a  tune  on  a  sort  of  electrical  organ,  not,  it 
is  true,  very  remarkable  for  the  fine  quality  of  its  tones.  I  now 
place  in  series  with  the  primary  an  electro-magnet  similar  in  con- 
struction to  one  of  those  used  by  Dr.  Elisha  Gray  in  his  electro- 
harmonic  telegraph  years  'ago.  This  magnet  is  fastened  to  the 


r6  ELECTRICITY  AT   HIGH   PRESSURES. 

bottom  of  a  resonating  box.  I  adjust  the  pitch  of  the  interruptions 
until  I  get  the  condition  of  resonance,  and  a  very  loud  sound 
testifies  not  only  to  the  vigor  of  the  interruptions  but  to  the  energy 
going  into  sound  waves.  We  may  have  here  an  electrical  substitute 
for  a  steam  whistle  or  fog  siren.  Even  for  signaling  on  moving 
trains,  electricity  need  not,  it  is  evident,  be  dependent  upon  com- 
pressed air. 

A  form  of  induction  coil  depending  upon  a  very  high  rate  of 
change,  as  well  as  upon  the  transforming  ratio  of  primary  to  sec- 
ondary windings,  is  the  well-known  high  frequency  coil.  In  a  sense, 
the  ordinary  induction  coil,  with  condensers,  as  above  stated,  is  a 
high  frequency  apparatus,  as  the  breaking  of  the  primary  circuit 
charges  the  condenser  across  the  break,  then  in  discharging  gives  a 
relatively  high  frequency  oscillation  through  the  primary  circuit. 
If  separate  means  be  employed  to  charge  the  condenser  to  a  high 
potential,  and  it  be  permitted  to  discharge  over  a  spark  gap  through 
a  few  turns  of  wire,  well  insulated,  we  have  a  high  frequency  dis- 
charge in  these  turns.  The  lower  the  capacity  of  the  condenser 
and  the  less  the  inductance  of  the  circuit,  the  frequency  will  be 
greater,  according  to  the  law  enunciated  by  Lord  Kelvin. 

In  1889,  Professor  Henry  A.  Rowland  of  Johns  Hopkins  Uni- 
versity, in  a  discourse  at  the  annual  meeting  of  the  American  Insti- 
tute,* drew  attention  to  the  high  frequency  effects,  and  employed 
experimentally  a  Ruhmkorff  coil,  the  discharge  of  the  secondary  of 
which  charged  some  Leyden  jars  or  condensers  Cl  (Fig.  9),  which 
were  discharged  over  a  spark  gap,  a,  through  a  few  turns  of  wire 
carried  on  a  frame  as  a  large  open  coil  with  an  air  core  as  at,  A,  in 
the  figure.  Opposite  to  this  and  parallel  in  plane,  that  is  in 
inductive  relation  to  it,  was  another  coil,  JB,  acting  as  a  secondary 
circuit  receiving  the  high  frequency  induction,  and  between  the 
terminals  of  which  at  b,  sparks  were  discharging  at  every  discharge 
of  the  jars  through  the  primary.  He  showed  also  that  by  attuning 
the  two  circuits,  as  by  connecting  jars  or  condensers,  <72,  to  the 
secondary  circuit,  B,  the  results  were  such  that  the  frames  or  coils 
could  be  placed  far  apart  without  preventing  the  inductive  action  of 
the  high  frequency  currents.  Professor  Rowland  had  all  of  the 
elements  of  a  high  frequency  coil  except  the  step  up  ratio  between 
primary  and  secondary.  As  far  back  as  1877  it  was  my  custom  in 
lecturing  on  Electricity  to  show  the  effects  of  the  discharge  of 
Leyden  jars  through  the  primary  of  an  ordinary  induction  coil  upon 

*  "  On  Modern  Views  with  Respect  to  Electric  Currents,"  pages  344,  345,  346, 
Transactions  of  American  Institute  of  Electrical  Engineers. 


ELECTRICITY   AT   HIGH    PRESSURES.  I/ 

the  secondary  as  a  step  up  apparatus,  and  the  reverse;  having,  in 
fact,  received  the  first  suggestion  of  electric  welding  while  discharg- 
ing a  battery  of  Leyden  jars  through  the  fine  wire  secondary,  while 
noting  the  extremely  heavy  current  set  up  in  the  primary  circuit. 
By  abolishing  the  iron  core  and  winding  only  a  single  layer  of 


FIG.  9. 

secondary  upon  an  insulating  frame  placed  inside  of  a  primary 
layer  of  but  few  turns,  and  immersing  the  whole  in  a  tank  of  oil,  we 
were  enabled  to  produce  the  now  well-known  high  frequency,  high 
potential  coil.  It  uses  an  alternating  current  of  ordinary  frequency 
with  a  step  up  transformer  to  charge  the  condensers,  sometimes 
with  a  strong  air  blast  blowing  upon  the  spark  gap  to  insure  regu- 


18  ELECTRICITY   AT   HIGH   PRESSURES. 

larity  of  discharges.  Professor  Rowland  had  employed  the  ordinary 
induction  coil  for  charging  his  condensers  and  an  ordinary  spark 
gap.  Tesla  employed  a  high  frequency  dynamo  to  charge  his  con- 
densers, and  a  blow-out  magnet  over  the  spark  gap,  while  in  his 
high  frequency  coil  he  at  first  retained  the  iron  core,  discarding 
it  later.  A  well-arranged,  high  frequency,  high  potential  coil  is 
adapted  to  reproduce  many  of  the  well-known  beautiful  effects  of 
Ruhmkorff  coils  using  a  rapid  break,  but  with  increased  brilliancy 
and  effect,  while  other  actions  are  peculiar  to  the  very  rapidly 
reversed  discharges  of  the  high  frequency  order.  The  construc- 
tion of  high  frequency,  high  potential  apparatus  may,  of  course,  be 
greatly  varied.  As  an  example  of  one  form  which  has  been  em- 
ployed by  me,  I  may  here  insert  a  description  of  an  apparatus 
giving  3o-inch  sparks  between  its  terminals. 

A  step  up  transformer,  whose  secondary  gives  20,000  volts  alter- 
nating current,  is  connected  to  charge  the  condensers,  the  discharge 
from  which  passes  by  air  gaps  through  the  primary  coil  of  the  high 
frequency  apparatus.  This  primary  consists  of  ten  turns  of  wire 
wound  on  a  wooden  frame.  The  conductor  is  of  two  No.  6  wires 
placed  side  by  side.  This  open  coil  is  18  inches  long  and  i$$4 
inches'  diameter.  Its  resistance  is  .0088  ohm  and  inductance  .0076 
millihenry.  The  secondary  coil  has  396  turns  of  No.  26  wire, 
wound  as  a  single  layer  in  notches  on  a  hard  rubber  frame,  the 
wires  being  spaced  apart  to  form  a  coil  18  inches  long.  The  di- 
ameter of  the  secondary  is  12  inches  and  the  weight  of  the  wire 
about  one  pound.  The  wire  of  the  secondary  coil  layer  equals  1250 
feet,  representing  a  resistance  of  41.6  ohms  and  inductance  of  25.2 
millihenrys.  These  coils  are  immersed  and  supported  concentrically 
in  a  vat  of  oil,  and  the  secondary  has  its  terminals  carried  to  the 
brass  rods  and  balls  which  form  the  discharge  terminals  of  the 
apparatus. 

There  are  used  two  condensers,  all  or  portions  of  which  may  be 
connected  for  the  primary  coil  discharge,  each  contained  in  a  box  7 
inches  x  15^  inches  inside,  and  17^  inches  deep.  Each  box  con- 
tains 84  built  up  mica  sheets  15  in.  x  15  in.  and  .075  in.  thick; 
42  of  these  are  coated  with  tin  foil  10  x  1 1  inches—  1 10  sq.  in.  Effec- 
tive foil  surface=45io  sq.  in.  The  condensers  are  immersed  in  oil 
in  the  boxes.  The  capacity  of  each  condenser  box  is  about  .03 
microfarad. 

In  the  apparatus  which  gave  64-inch  sparks  between  the  termi- 
nals, the  primary  coil  has  a  length  of  28  inches  and  diameter  of  22 
inches.  It  has  15  turns  of  double  No.  6;  85  feet  of  wire,  doubled. 


ELECTRICITY   AT    HIGH    PRESSURES.  19 

Its  resistance  is  .0147  ohm,  and  inductance  .09  millihenry.  The 
secondary  coil  length  is  28  inches,  its  diameter  being  17  .inches. 
The  wire. forms  one  layer  with  the  turns  spaced  apart  and  carried  in 
notches  in  a  hard  rubber  frame,  580  turns  in  the  layer.  The  wire  is 
No.  26,  about  2^  Ibs.  or  2600  feet  total  length. 

The  condensers  used  with  the  primary  were  in  three  boxes,  the 
dielectric  being  of  mica  plates  and  oil.  When  all  were  in  use  the 
capacity  was  .046  microfarad.  These  were  charged  by  a  large  step 
up  transformer  to  30,000  volts  and  discharged  across  air  gaps 
through  the  primary.  An  air  blast  was  kept  blowing  on  the  gaps. 
The  greatest  distance  at  which  it  was  possible — on  account  of  the 
construction — to  set  the  terminals  was  64  inches,  which  was  crossed 
with  ease.  The  probability  is  that  if  the  terminals  could  have  been 
more  widely  separated,  longer  discharges  could  have  been  obtained. 
The  current  used  in  the  primary  of  the  step  up  transformer  being 
of  125  cycles,  there  were  at  least  250  of  the  64-inch  discharges  in 
each  second. 

We  should  be  cautious  in  accepting  some  statements  in  regard  to 
high  frequency  currents.  It  has  been  claimed  that  insulators  are 
conductors  for  such  currents,  and  experiments  have  been  shown  in 
illustration  thereof.  Thus  the  two  terminals  of  a  high  frequency 
coil,  when  placed  in  connection  with  metal  plates  on  each  side  of  a 
hard  rubber  sheet,  seem  to  be  short  circuited,  or  if  the  sheet  be  double 
and  the  two  parts  be  separated  a  short  space,  an  intense  blue  dis- 
charge is  seen  in  the  space  between  the  sheets  so  separated,  as  if  the 
current  got  through  the  dielectric.  Even  glass  in  the  same  way  has 
been  regarded  mistakenly  as  conducting  the  discharges.  It  is,  of 
course,  only  an  effect  of  capacity  which  gave  rise  to  the  miscon- 
ception. 

The  insulating  power  of  oil  for  high  frequencies  is  as  much  as  ten 
to  twenty  times  what  it  is  for  low  frequencies,  and  it  is  possible  that 
many  other  insulators  show  similar  relations. 

Some  years  ago  1  investigated  some  of  the  properties  of  oil  as  an 
insulator.  I  found  that  for  certain  kinds  of  currents,  such  as  high 
frequency  currents,  oil  was  from  forty  to  sixty  times  as  good  an 
insulator  as  air;  that  is,  a  5o-inch  spark  would  be  insulated  by 
using  an  inch  of  oil.  We  had  terminals  in  air  fifty  inches  apart,  and 
other  terminals  in  oil  one  inch  apart,  and  in  this  case  there  was  an 
even  chance  of  its  jumping  under  the  oil  or  through  the  air.  We 
thought  this  fact  favored  the  use  of  oil ;  but  it  turned  out  when  we 
tried  these  same  experiments,  or  similar  ones,  with  ordinary  dis- 
charges of  low  frequency,  that  the  very  same  oil  was  only  about  two 


20  ELECTRICITY   AT   HIGH    PRESSURES. 

and  one-half  times  as  good  an  insulator  as  air — that  a  two  and  one- 
half  inch  spark  was  all  that  would  be  insulated  by  one  inch  of  oil, 
varying  according  to  the  condition  or  quality  of  the  oil,  which  in- 
sulation was  nothing  as  compared  with  the  insulating  power  for  the 
higher  frequencies.  We  found  also  a  most  striking  thing  with  points. 
If  we  put  two  points  opposite  to  each  other  in  air  they  of  course 
facilitate  the  discharges,  but  we  found  that  points  under  oil  were 
far  better  than  polished  balls,  and  polished  balls  better  than  plates; 
that  oil  between  plates  was  broken  down  easily.  It  would  seem  that 
to  get  good  insulation  under  oil  we  would  have  to  make  the  con- 
ductor full  of  points.  Whether  this  experimental  fact  would  hold 
good  when  a  great  many  points  were  clustered  together,  I  do  not 
know.  We  used  single  points. 

That  brilliant  physicist  Plante,  whose  name  is  so  well  known  in 
connection  with  the  early  Plante  cell  or  accumulator,  invented  what 
he  called  a  rheostatic  machine,  consisting,  in  substance,  of  a  set 
of  condensers  charged  in  multiple  (as  from  a  great  number  of 
battery  cells  connected  in  series  and  giving,  say,  5000  volts),  with 
commutating  or  switching  apparatus  for  connecting  the  condenser 
plates  in  series  or  in  cascade,  thus  reducing  the  capacity  of  the 
series  set  and  raising  the  terminal  potential  in  proportion.  If  ten 
such  condensers  were  used  in  the  apparatus,  and  the  charging 
potential  were  5000  volts,  then  the  series  connection  would  yield 
discharges  of,  approximately,  50,000  volts. 

Professor  Trowbridge  of  Harvard  has  in  recent  years  greatly 
magnified  the  effects  of  the  Plante  machine.  He  has  arranged 
small  storage  cells  to  be  charged  in  sets  or  In  multiple  by  a  dynamo, 
and  then  connected  in  series  so  as  to  give  20,000  volts  or  more  of 
continuous  potential  at  the  terminals.  With  this  battery  of  rela- 
tively high  potential  he  charges  in  multiple  a  range  of  condensers 
consisting  of  glass  plates  coated  on  the  two  sides  with  tinfoil,  ex- 
cept near  the  edges.  By  a  large  shifting  frame  worked  by  a  lever 
he  throws  the  connection  of  the  condensers,  after  charging,  into 
series  order  or  cascade.  The  end  condenser  foils  of  the  set  are 
connected  with  the  discharge  terminals  of  the  apparatus.  In  this 
way  he  obtains  with  great  ease  discharges  of  three  or  four  feet  in 
length,  and  of  great  brilliancy  and  beauty,  one  spark  at  each  throw 
of  the  commutator  lever.  With  a  large  machine  of  this  type  he  has 
obtained  sparks  of  seven  feet  in  length,  the  distance  covered  being 
in  fact  limited  by  too  close  proximity  of  the  walls  of  the  building, 
its  floor,  its  roof,  etc.,  and  not  by  the  real  capability  of  the 
apparatus,  which,  without  the  leakages  and  lateral  discharges, 


ELECTRICITY   AT   HIGH    PRESSURES.  21 

would  be  capable  of  producing   potential   differences  of  3,000,000 
volts. 

It  was  while  studying  the  action  of  Professor  Trowbridge's  appara- 
tus that  it  occurred  to  me  to  dispense  with  the  large  series  of  battery 
cells  used  in  charging,  and  employ  instead  thereof  the  tops  of  the 
waves  of  a  high  potential  alternating  current  suitably  delivered  in 
one  direction.  I  was  thus  enabled  to  produce  a  new  machine  for 
obtaining  high  potential  discharges.  It  consists  partly  of  a  motor- 
dynamo,  such  as  is  obtained  by  taking  an  ordinary  continuous  cur- 
rent motor  and  tapping  the  winding  so  as  to  obtain  alternating  cur- 
rents. To  this  end  two  of  the  commutator  segments  or  leads  are 
connected  to  a  pair  of  insulated  metal  rings  on  the  shaft.  Brushes 
resting  on  these  rings  yield  alternating  currents  of  a  periodicity  de- 
pending upon  the  speed  and  number  of  field  poles.  This  machine 
may  be  driven  by  continuous  current,  or  as  a  synchronous  alternat- 
ing current  motor  after  starting,  or  it  may  be  driven  by  mechanical 
power  as  a  self-exciting  alternating  current  dynamo.  The  alternat- 
ing current  brushes  are  connected  to  the  terminals  of  the  primary 
winding  of  a  step  up  transformer,  giving  in  the  secondary  a  poten- 
tial of,  say,  10,000  to  15,000  volts.  Driven  by  the  shaft  of  the 
machine  is  a  frame  of  insulating  material  as  wood,  having  at  one 
side  a  pair  of  metal  strips  which  periodically  connect  the  high 
potential  secondary  terminals  of  the  step  up  transformer  to  the 
plus  and  minus  foils  of  a  set  of  condensers  (eleven  glass  plates 
coated  with  tinfoil)  in  parallel.  These  connections,  to  avoid  noise 
and  friction  are  made  without  actual  contact,  that  is,  over  a  small 
spark  gap.  The  revolving  frame  is  so  adjusted  that  the  charging 
shall  be  completed  only  at  the  tops  of  the  waves,  and  thus  a  high 
potential  be  available  for  charge.  Moreover,  in  the  particular 
machine  before  you  (Fig.  10),  the  tops  of  only  every  third  alternat- 
ing wave  of  same  polarity  are  employed,  and  thus  the  condenser 
plates  or  foils  are  always  given  the  same  polarity.  In  this  way  the 
chopped  up  alternating  discharge  becomes,  for  charging  the  conden- 
sers, a  substitute  for  a  high  potential  battery.  The  revolving  frame 
carrying  the  charging  strips  also  carries  a  set  of  series  connectors 
whereby,  after  the  charging  strips  have  withdrawn  from  proximity 
to  the  stationary  contacts  led  from  the  condenser  foils,  these  con- 
tacts are  connected  in  series  as  in  the  Plante  or  Trowbridge  ma- 
chines, and  the  terminals  or  end  foils  discharge  across  a  wide  air 
gap  at  high  potentials,  giving  a  rapid  series  of  sparks  of  about  12 
inches  in  length  in  the  apparatus  here  shown.  The  length  of  spark 
is  governed  by  the  number  of  condenser  plates  and  the  potential  of 


22  ELECTRICITY   AT   HIGH    PRESSURES. 

the  charging  current.  This  may  be  made  to  correspond  with  the 
maximum  potential  of  the  wave  in  the  secondary  of  the  high  poten- 
tial or  step  up  transformer.  I  find  shellacked  wood  sufficiently  in- 
sulating for  the  parts  of  the  machine  outside  of  the  condensers  and 
metal  connections. 

The  machine  has  been  further  developed  and  improved  by  me 
until  it  actually  becomes  a  practical  substitute  for  a  static  machine, 
independent  of  the  weather.  It,  therefore,  needs  no  outer  case  nor 
means  for  [drying.  I  have  added  a  simple  attachment  whereby 


FIG.  10. 

Leyden  jar  batteries  may  be  charged  or  a  stream  of  thin  sparks  ob- 
tained, and  with  this  attachment  the  machine  may  be  used  to  excite 
the  sectors  of  large  influence  machines,  if  desired,  in  all  states  of 
the  wreather. 

This  attachment  consists  of  a  revolving  connector  covering  a 
wide  gap  between  one  of  the  terminals  or  end  condenser  foils  and  a 
stationary  insulating  ball  or  conductor.  This  connector  itself,  con- 
sisting of  a  pair  of  balls  or  rounded  surfaces  connected  by  a  wire, 
is  insulated  and  synchronously  bridges  the  gap  of  several  inches  be- 
tween the  end  condenser  terminal  and  the  insulated  ball.  The  time 


ELECTRICITY   AT   HIGH    PRESSURES.  23 

of  making  this  connection  coincides  with  the  series  connection 
made  by  the  revolving  frame.  The  insulated  ball  or  conductor 
thus  is  synchronously  charged,  while  the  opposite  terminal  of  the 
apparatus  may  be  put  to  earth.  The  ball  may  be  made  either  posi- 
tive or  negative  by  changing  the  alternating  current  connections 
from  the  collector  rings  to  the  primary  of  the  step  up  transformer 
or  in  other  ways.  A  Leyden  jar  battery  or  condenser  may  be 
charged  by  connecting  its  interior  coating  with  the  insulated  ball 
and  its  exterior  to  earth,  or  to  the  opposite  terminal  of  the  appara- 
tus, and  from  the  charged  jar  condenser  a  string,  dipped  in  very 
weak  acid  or  rubbed  in  graphite,  may  be  made  the  means  for  convey- 
ing the  jar  charge  slowly  to  a  prime  conductor  for  weak  or  thin 
sparks,  whereby  the  effects  of  a  static  machine  may  be  closely 
reproduced. 

The  whole  apparatus  combines  so  many  features  of  transform- 
ation of  energy  as  to  become  a  highly  instructive  apparatus  for 
schools,  and  its  capabilities  are  quite  varied,  as  can  be  readily 
understood.  I  call  it  my  Dynamo-Static  Machine. 

I  may  mention  as  an  interesting  fact  that  the  new  machine  may 
be  used  to  charge  a  set  of  condensers  to  a  high  potential  in  a 
second  rheostatic  machine,  after  which  series  connections  being 
made,  as  in  Professor  Trowbridge's  apparatus,  a  second  multiplica- 
tion of  potentials  would  result.  Very  high  potentials  may  thus 
be  obtained. 

There  are  other  parts  of  the  subject  which  can  only  be  touched 
upon  now,  as,  for  example,  the  varied  uses  to  which  electricity  at 
high  pressures  is  put,  the  question  of  insulation,  dielectric  strength, 
striking  distance,  leakage,  etc.,  etc.  I  may  call  attention  also  to 
the  difficulty  experienced  in  measurement  of  high  potentials,  owing 
to  the  striking  distance  demanding  that  the  parts  of  measuring 
instruments  for  high  potential  be  widely  separated.  I  have  at- 
tempted to  overcome  this  difficulty  by  sealing  in  a  glass  bulb  the 
parts  of  a  small  electrostatic  voltmeter  and  in  a  vacuum  so  high  as 
to  be  non-conducting,  in  which  case  the  parts  may  be  closely  placed 
with  the  result  of  greatly  increased  torque  and  freedom  from  short 
circuits. 

When  two  terminals  as  of  a  circuit  are  at  high  differences  of  poten- 
tial, a  discharge  or  neutralization  across  the  gap  may,  as  is  well 
known,  take  place  and  may  give  rise  to  a  variety  of  effects  depend- 
ing upon  conditions.  The  striking  distance  in  air,  or  the  distance 
over  which  a  -spark  or  arc  discharge  can  be  formed,  is  itself  very 
variable.  When  the  terminals  are  polished  balls  and  the  parts  of 


24  ELECTRICITY   AT   HIGH    PRESSURES. 

the  circuit  discharging  have  some  capacity,  the  spark  is  sharp  and 
distinct;  if,  however,  after  this  initial  spark  the  high  pressure  be 
maintained  an  arc  or  flame  takes  the  place  of  the  first  spark,  and 
may  continue  indefinitely,  as  in  electric  power  transmissions  in  the 
case  of  a  short  circuit  occurring.  It  is  important  to  note  also  that 
the  effect  of  heating  the  terminals  is  to  increase  the  striking  dis- 
tance to  a  marked  degree.  Apparatus  at  high  potentials  becomes 
more  safe  from  discharge  if  the  parts  are  not  overheated. 

A  blast  of  air  or  a  magnetic  field  between  the  discharging  ter- 
minals tends  to  stop  any  arc  by  lengthening  it  so  that  it  breaks,  but 
neither  of  them  has  any  effect  upon  the  striking  distance  proper 
nor  upon  the  effect  of  hot  terminals,  so  far  as  is  determined.  Ter- 
minals between  which  an  arc  has  been  playing  may  become  so 
heated  that,  although  they  are  far  enough  apart  not  to  be  crossed 
by  a  given  potential  when  cool,  the  discharge  easily  renews  itself, 
and  does  this  repeatedly;  in  spite  of  extinction  by  a  magnetic 
field,  for  example. 

With  alternating  currents  such  as  are  used  in  power  transmission 
at  10,000  to  50,000  volts,  it  is  found  that  the  striking  or  arcing  dis- 
tance between  needle  points  may  be  used  as  a  fair  measure  of  the 
maximum  potential  difference.  Mr.  C.  P.  Steinmetz  has  investi- 
gated this  matter  up  to  over  200,000  volts,  and  has  published  his 
results,  together  with  a  curve  showing  the  relation  of  striking  dis- 
tance to  potential  in  such  cases.  In  such  a  curve,  if  ordinates  rep- 
resent striking  distances  and  abscissas  potentials  of  discharge, 
the  curve  on  leaving  the  origin  is  at  first  considerably  convex  to  the 
base;  above  10,000  to  20,000  volts  becomes  nearly  straight;  and  at 
about  200,000  volts  tends  upward  more  rapidly.  The  general  tend- 
ency is  to  a  more  rapid  increase  in  striking  distance  than  in  direct 
proportion  to  the  voltage,  although,  for  a  considerable  range,  as 
from  about  20,000  to  200,000,  the  two  are  nearly  proportional. 

I  have  already  mentioned  the  anomalous  condition  as  to  striking 
distance  under  oil  when  points  were  compared  with  balls  and  plates, 
a  condition  the  reverse  of  that  which  obtains,  of  course,  in  air  and 
gases  generally. 

It  would  seem  that  a  perfect  vacuum  might  require  an  infinite 
potential  to  send  a  discharge  even  over  quite  a  small  gap  between 
terminals  in  the  vacuous  space.  It  would  take  us  too  far  afield  to 
consider  the  very  beautiful  and  instructive  effects  produced  by  dis- 
charges in  high  vacua  and  in  spaces  not  so  completely  exhausted. 
This  is  a  field  of  scientific  investigation  which  has  since  the  early 
experiments  of  Hittorf  and  Crookes  yielded  a  rich  harvest  of 


ELECTRICITY   AT    HIGH    PRESSURES.  2$ 

scientific  facts  and  to  which  a  renewed  interest  has  been  in  recent 
years  imparted  by  the  work  of  Lenard,  and  Roentgen,  and  that  of 
many  others,  among  them  Professor  J.  J.  Thomson's  investigations. 
Much  more  is  doubtless  to  be  learned  in  this  field  of  work. 

When  a  solid  or  plastic  dielectric  is  used  as  insulation  around  a 
conductor,  or  between  two  conductors  with  high  pressure  stress- 
ing the  dielectric  layer,  discharge  may  take  place  by  leakage  or  by 
puncture.  The  latter  means  usually  a  breakdown;  the  former,  if 
not  great,  may  not  be  serious.  The  higher  the  temperature  of  the 
insulating  layer  the  less,  as  a  rule,  is  its  insulating  power. 

It  is  well  known  that  all  insulators  without  exception  are  either 
destroyed  by  decomposition  or  become  conducting  at  high  tem- 
peratures. This  fact  has  been  commonly  recognized  for  many 
years  past,  although,  curiously  enough,  it  has  been  put  forward  as 
new  in  connection  with  the  much-talked-of  Nernst  lamp.  So  also 
the  dielectric  strength,  or  power  to  resist  puncture  under  electric 
stress,  is  generally  weakened  by  increase  of  temperature. 

When  the  electric  stresses  are  alternated  rapidly,  as  in  condensers 
used  on  the  higher  periodicity  alternating  circuits  or  with  high 
frequency  apparatus,  the  presence  of  air  or  gas  bubbles  in  the  die- 
lectric layer  is  usually  very  objectionable,  not  only  on  account  of 
the  loss  of  energy  involved,  but  from  the  great  risk  of  puncture, 
provoked,  it  may  be,  by  heating.  Each  little  bubble  of  air  inclosed 
in  the  dielectric,  or  between  it  and  the  conducting  metal,  is  filled 
with  blue  discharges  which  waste  energy  and  produce  local  heat 
and  liability  to  puncture. 

Some  dielectrics,  even  when  solid,  seem  to  be  unsuited  as  insula- 
tors where  the  frequency  is  high.  In  my  experience  glass  is  easily 
broken  down  in  such  work  unless  kept  under  low  stresses. 

It  would  be  an  interesting  study  to  follow  the  effects  of  high 
periodicity  in  the  use  of  such  dielectrics  as  glass  as  compared  with 
oil  and  the  like,  increase  of  frequency  seemingly  conducing  to  the 
destruction  of  the  one  and  adding  value  to  the  other. 

The  phenomenon  of  soakage  may  have  more  or  less  to  do  with 
these  different  actions.  Some  specimens  of  lightly  glazed  earthen- 
ware  exhibit  very  striking  power  of  absorbing  a  charge.  I  have 
found  dishes  which,  when  subjected  to  the  high  potential  discharges 
of  a  powerful  frequency  coil,  or  even  to  those  of  a  Holtz  machine, 
would  continue  to  sparkle  in  the  dark  for  a  considerable  time  after- 
ward, and  which  would  yield  innumerable  short  sparks  to  the  finger 
tip  from  various  parts  of  the  surface. 

The  experiment  is  both  curious  and  striking,  being  made  all  the 


26  ELECTRICITY   AT   HIGH    PRESSURES. 

more  so  by  placing  a  layer  of  oil  on  the  dish  before  subjecting  it,  to 
the  discharge.  The  oil  afterward  moves  about  on  the  dish,  owing 
to  the  charge  working  out  from  its  interior. 

There  can  be  no  doubt  that  porcelain  insulation  which  is  not  so 
thoroughly  vitrified,  as  to  be  non-porous,  will  behave  in  the  same 
way,  and  will  in  consequence  be  easily  broken  down  upon  the  con- 
tinued application  of  high  electric  pressures,  though  at  first  it  may 
appear  to  have  a  sufficient  dielectric  strength. 

The  phenomenon  of  surface  creeping,  or  imperfect  conduction  over 
surfaces  of  insulating  material,  is  naturally  of  very  great  importance 
in  limiting  the  insulation  which  can  be  maintained  for  high 
pressures.  When  a  condenser  such  as  a  Leyden  jar  is  charged 
beyond  a  certain  potential  difference  between  its  foils,  we  note  the 
appearance  of  thin  purplish  sparks  at  the  edges  of  the  foils,  which 
sparks  are  all  the  more  marked  when  the  charging  current  is  alter- 
nating or  periodic.  The  tendency  of  any  electric  charge  is  to 
increase  the  capacity  by  spreading  or  other  action.  This  spread- 
ing of  a  charge  at  high  potentials  is  in  fact  one  of  the  most 
important  factors  in  limiting  the  pressure  which  may  be  maintained. 
It  may  extend  in  the  case  of  the  Leyden  jar  condenser  so  far  over 
the  glass  above  the  coating  as  to  cause  discharge.  This  fact  is,  of 
course,  well  known.  The  same  action,  however,  must  be  recognized 
as  occurring  at  the  edges  of  the  foils,  even  when  condensers  are 
immersed  in  oil  or  surrounded  by  insulating  liquids  or  solids.  Its 
effect,  however,  under  such  conditions  may  be  quite  serious.  I 
have  noticed,  for  instance,  that  condensers  consisting  of  coated 
glass  or  mica  plates,  subjected  to  alternating  current  charge  and 
discharge  at  about  20,000  volts  while  immersed  in  oil,  are  subject 
to  deterioration  at  the  edges  of  the  tinfoil  coatings  to  such  an 
extent  that  the  oil  is  partly  decomposed  by  minute  sparks,  and  the 
solid  dielectric  cut  away  or  etched  at  the  same  time.  This  eventu- 
ally leads  to  breakdown. 

In  high  pressure  work,  particularly  in  that  involving  large 
amounts  of  energy  in  transfer,  as  in  power  transmissions,  the  avoid- 
ance of  dust  or  dampness  upon  the  surfaces  of  insulation,  over  which 
a  discharge  or  leak  may  form  by  creeping,  is  of  the  greatest  impor- 
tance. So,  also,  the  avoidance  on  either  side  of  a  circuit  of  sharp 
projecting  edges  or  points  near  metal  connected  to  ground,  or  to 
the  opposite  sides  of  the  system,  is  imperative.  To  this  end  the 
high  pressure  conductors,  where  near  together,  are  incased  as  much 
as  possible  in  a  sufficient  thickness  of  insulator  to  withstand  from 
two  to  three  times  the  normal  pressure  for  a  certain  time,  such  as 


ELECTRICITY   AT   HIGH    PRESSURES.  2/ 

ten  minutes,  selected  for  the  duration  of  test.  Where  the  con- 
ductors are  not  incased,  they  are  given  a  large  separation  far 
in  excess  of  the  normal  striking  distance  between  points  at  the 
potential  difference  involved. 

The  practice  of  subjecting  electric  apparatus  to  tests  with  pres- 
sures much  higher  that  those  of  normal  use  is  now  quite  general, 
and  conduces  more  than  any  other  thing  to  security  from  break- 
downs due  to  loss  of  insulation.  Apparatus  intended  to  work 
normally  at  40,000  volts  may  thus  have  to  stand  test  pressures 
which  involve  actual  striking  distances  of  several  inches. 

Besides  the  enormous  extension  of  systems  of  power  transmission 
by  polyphase  currents  involving  pressures  of  from  five  thousand  to 
forty  thousand  volts,  measured  as  alternating  current,  which  really 
involve  a  much  higher  pressure  depending  upon  the  form  of  the 
wave,  a  number  of  other  uses  of  high  pressure  electricity  may  be 
briefly  alluded  to. 

The  production  of  Roentgen  rays  involves  pressures  ranging  from 
about  40,000  to  150,000  volts,  but  the  current  is  of  course  insignifi- 
cant. The  higher  the  vacuum  in  the  Crookes  tube  the  higher  the 
potential  demanded  to  pass  the  discharge,  though  this  condition 
may  be  complicated  by  other  phenomena.  I  have  in  fact  frequently 
found  a  tube  in  work  to  apparently  pass  instantly  from  the  condition 
regarded  as  indicating  lower  vacuum  to  one  of  very  high  vacuum 
and  the  reverse.  Certain  constructions  of  tube  permit  the  operator 
to  control  it  instantly  or  choose  in  what  way  the  tube  be  worked. 
The  Roentgen  rays,  emitted  from  tubes  demanding  the  highest 
potentials  to  work  them,  are  recognized  as  the  more  penetrating  or 
the  less  easily  absorbed  in  metals  or  other  substances. 

It  is  indeed  an  interesting  speculation  as  to  what  might  be  the 
character  of  rays  produced  by  potentials  of  a  million  volts,  for 
example,  passing  through  tubes  the. vacua  in  which  are  made  pro- 
portionately high.  If  the  rule  holds  good,  such  rays  should  pass 
freely  through  most  metals  in  considerable  thickness;  they  should 
hardly  affect  fluorescent  materials  such  as  calcium  tungstate  or  bar- 
ium platinocyanide;  and  the  photographic  plate  should  stop  such  a 
small  percentage  as  to  remain  practically  unaffected.  The  test  of 
their  presence  would  probably  be  their  power,  if  still  existent,  of 
ionizing  gases  and  causing  electric  conduction  or  convection 
through  them. 

An  old  use  of  high  pressure  electricity  to  which  more  and  more 
attention  appears  to  be  given  is  that  of  generating  ozone  in  ozon- 
izers.  This  is  so  well  known  that  it  is  needless  to  dwell  thereon. 


28  ELECTRICITY   AT   HIGH    PRESSURES. 

Apparatus  for  cutting  glass  by  spark  perforation  in  the  line  of 
a  desired  fracture  has  been  recently  worked,  and  furnishes  a  curious 
instance  of  the  application  of  high  pressures  accompanied  by  cur- 
rents or  discharges  so  small  as  to  be  almost  insignificant. 

The  development  of  the  wireless  telegraph  in  the  hands  of  Signer 
Marconi  is  another  example  which  tends  to  a  renewed  interest  in 
electrical  apparatus  for  developing  high  pressure  discharges. 

The  possibility  of  signaling  without  wires  and  through  the 
agency  of  electric  radiation  or  ether  waves  was,  I  believe,  pointed 
out  by  Hertz  himself  in  connection  with  his  beautiful  researches  in 
that  field. 

The  radiations  sent  out  and  picked  up  by  the  vertical  wire  of 
Marconi,  and  the  response  of  the  sensitive  coherers  of  Lodge  and 
Branly  have  already  been  the  means  of  communication  over  dis- 
tances so  great  as  to  leave  room  for  hope  that  hundreds  of  miles 
may  not  be  insurmountable  by  this  latest  important  development 
in  the  field  of  electricity  at  high  pressures.  It  was  early  seen  that 
the  Hertz  experiments  would  probably  make  it  possible  to  signal 
at  sea,  despite  storm  or  fog,  and  this  expectation  has  been  fully 
realized.  The  next  ten  years  will  doubtless  witness  a  very  great 
development  in  this  fascinating  field  and  mankind  will  be  greatly 
benefited. 

There  is  very  much  yet  to  be  learned  concerning  those  vast 
natural  exhibitions  of  electrical  actions  at  high  pressures  which 
fill  us  with  wonderment  and  awe.  I  allude  especially  to  the  light- 
ning and  the  aurora.  And  what  have  we  to  say.  about  that  mystery 
"  globular  lightning"  ?  Its  existence  seems  to  be  so  well  attested 
that  we  can  scarcely  doubt.  Personally,  I  came  so  near  seeing  it 
in  one  instance  that,  if  my  eyes  had  been  turned  toward  the  north- 
west instead  of  to  the  northeast,  I  would  have  been  a  witness  of  it, 
for  a  companion  who  was  looking  the  other  way  did  see  it,  and 
called  my  attention  just  a  moment  too  late.  I  did  hear,  however, 
the  noise  of  the  explosion  of  the  ball  which  he  saw  slowly  fall. 
The  noise  was  not  thunder,  but  merely  a  single  explosion  without 
rumble,  and  resembled  the  boom  of  a  cannon  of  the  old  type.  It  is 
well  known  that  the  cause  of  the  rattle  and  roll  of  thunder  from 
ordinary  flashes  is  the  length  of  the  spark  discharge,  the  sounds 
from  the  various  twistings  and  angles  of  which  reach  the  ear  suc- 
cessively on  account  of  their  varying  distance  from  the  observer. 
From  this  it  follows,  by  the  way,  that,  as  with  light  in  the  case  of 
the  rainbow,  no  two  observers  receive  exactly  the  same  impression 
of  sound  in  thunder. 


ELECTRICITY   AT   HIGH    PRESSURES.  29 

Auroral  displays  are  shown  to  be  probably  dependent  upon  solar 
disturbances;  an  earthly  coronal  stream,  perhaps,  developing  in 
response  to  some  unusual  coronal  development  on  the  sun,  or  to 
some  vast  sun  spot  disturbance.  I  am  tempted  to  think  that  pos- 
sibly the  flame  gases  of  the  sun  actually  reach  the  upper  atmos- 
phere of  the  earth,  and  break  down  the  insulation  of  the  layers 
already  under  electric  stress,  or  themselves  bring  electricity  which 
disturbs  the  condition  of  our  air.  I  am  disposed  to  favor  the 
former  idea,  however,  on  account  of  the  temporary  character  of 
the  discharges.  The  earth  may  in  fact  be  brushed  by  an  invisible 
prolongation  of  a  coronal  streamer,  the  effect  of  which  acting  like 
ionized  gas,  or  flame  gases,  or  gases  through  which  an  electric  dis- 
charge has  been  passed,  is  to  make  the  upper  thin  air  conduct, 
and  relieve  its  accumulated  electric  stresses  in  an  hour  or  two,  after 
which  follows  a  period  of  comparative  quiescence.  Besides  the 
solar  corona,  the  streaming  light  of  the  comet's  tail  may  indicate 
electric  redistribution  as  the  comet  approaches  the  sun — a  view 
which  has  gained  ground  in  the  past  few  years;  and  there  are  not 
wanting  astronomers  who  suspect  that  some  of  the  light  of  nebulae 
may  be  electrical  in  its  origin,  or  similar  to  that  of  the  comet's  tail. 
Nature  does  things  on  a  grand  scale,  and  her  celestial  electrical 
manifestations  may  not  be  unlike  those  which  we  are  wont  to 
call  terrestial,  although  dependent  perhaps  upon  the  grander  actions 
outside  the  earth's  gas  envelope  at  even  higher  pressures. 


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